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Power off the machine and visually inspect the external encoder feedback cable from the encoder head to the servo amplifier JD1A or JD1B connector. Check for cuts, pinch points, excessive bending near connectors, or damage in cable tracks.

Lock out main power and wait 5 minutes for capacitor discharge before handling servo amplifier connections.

Disconnect and reseat the external encoder cable at both the encoder head and the servo amplifier JD1A/JD1B connector. Inspect connector pins for damage, bent pins, or contamination. Verify locking mechanism is fully engaged.

Use a digital multimeter to check cable continuity on all encoder signal lines (A+, A-, B+, B-, Z+, Z-, shield) and verify no shorts between signal pairs or to ground. Replace cable if any conductor shows >1 ohm resistance or shorts are detected.

After reconnecting, navigate to DIAGNOSIS screen > Servo and monitor DGN#300-#307 (encoder feedback signals) while manually rotating the axis. Verify pulse counts increment smoothly without erratic jumps or freezing.

With power on and alarm displayed, check the servo amplifier LED indicators. A-series and B-series amplifiers display specific fault codes. Note any flashing patterns or steady red LEDs indicating encoder fault (typically LED #4 or encoder-specific LED).

Navigate to DIAGNOSIS > Servo screen and check DGN#400-#407 (encoder power voltage). Verify the external encoder power supply reads between 4.75V-5.25V DC. Low voltage indicates amplifier power circuit failure or excessive current draw from damaged encoder.

Disconnect the external encoder cable at the encoder head. Measure resistance between encoder power pins (+5V to 0V) with multimeter. Typical reading should be 80-150 ohms depending on encoder model. <10 ohms indicates internal short; >1K ohms indicates open circuit. Replace encoder if out of range.

Measure with power OFF to avoid damaging encoder electronics.

If encoder appears functional, check for mechanical contamination. Remove encoder cover and inspect encoder disk/scale for oil, coolant, metal chips, or physical damage. Clean with lint-free cloth and isopropyl alcohol if contaminated. Verify encoder bearing has no excessive play or roughness.

Verify parameter setting for external encoder configuration. Check PARAM#1815 (encoder type setting) and PARAM#1826 (external encoder selection bit). Ensure bit #0 is set to 1 for external separate detector usage. Incorrect setting can cause signal interpretation errors.

Perform encoder signal substitution test. If available, temporarily swap the external encoder cable with a known-good axis using the same encoder type. If alarm 387 follows the amplifier, this confirms amplifier fault; if alarm stays with cable/encoder, confirms external device fault.

Only perform substitution on identical encoder types and compatible axes to prevent parameter conflicts.

Check for electrical noise interference. Navigate to DIAGNOSIS > Servo and monitor DGN#308-#315 (encoder error counter). If error counts increment rapidly or continuously, verify encoder cable shielding is grounded only at amplifier end, ensure separation from power cables, and check for nearby VFDs or welders causing EMI.

If all external components test good, replace the servo amplifier. Before replacement, record all axis parameters (PARAM#1800-#1899 series servo parameters). After amplifier replacement, restore parameters and perform encoder offset setup using SYSTEM > PMC > SETTING > separate detector adjustment procedure per maintenance manual.

Servo amplifier replacement requires proper parameter backup and encoder reference position re-establishment.